1. Field of the Invention
The present invention relates to an adapter power supply; and more particularly, to an adapter power supply estimating the load current of output power through the primary current of a transformer and varying the link voltage of DC power serving as an input power of a DC/DC converter according to variation of the estimated load current.
2. Description of the Related Art
An adapter power supply is widely used in electronic apparatuses such as a notebook PC, a display monitor, a cellular phone, and the like.
Generally, such an adapter power supply has a double-stage type structure consisting of an AC/DC converter taking exclusive charge of a power factor and harmonic restriction and a DC/DC converter capable of desired output voltage.
Unlike a characteristic of a general power supply in which an output voltage decreases as an output current increases, the adapter power supply is required to have a load characteristic in which the output voltage increases as the output current serving as the load current. For this, recently, there has been proposed a method of operating an LLC resonant converter in the vicinity of a resonant frequency at all times by varying an link voltage of a DC power serving as an input power of a DC/DC converter according to a load current by using a boost converter as an AC/DC converter and the LLC resonant converter as the DC/DC converter.
However, in the above-described method, since the load current was detected by inserting a resistor between a secondary output terminal of the transformer of the DC/DC converter and a load, power was lost due to a current measurement resistance. A link voltage variation signal generated from the current measurement resistance has to compensate for an error signal inputted into a gain/integral controller positioned a secondary terminal of the DC/DC converter and should control the link voltage by transmitting a signal even to a controller of the AC/DC converter, an additional Opto Coupler in addition to an Opto coupler for controlling an output voltage of the DC/DC converter have to be used. Such an added Opto coupler and an additional circuit for actuating the same an the like increase the volume of the adapter power supply, thereby making it difficult to manufacture a high-density adapter power supply.